This invention relates generally to turbine engines, and more specifically to blades used in axial compression systems.
In a gas turbine engine, air is pressurized in a compression module during operation. The air channeled through the compression module is mixed with fuel in a combustor and ignited, generating hot combustion gases which flow through turbine stages that extract energy therefrom for powering the fan and compressor rotors and generating engine thrust to propel an aircraft in flight or to power a load, such as an electrical generator.
The compression system includes a rotor assembly comprising a plurality of rotor blades extending radially outward from a disk. More specifically, each rotor blade has a dovetail which engages with the disk, a platform forming a part of the flow path, and an airfoil extending radially from the platform to a tip. The platform may be made integral to the blade or, alternatively, made separately.
In some designs, the rotor blade, especially those in a fan rotor and the front stages of a multistage compression system, have a pair of circumferentially extending shrouds on the airfoil, one projecting from the pressure surface and one projecting from the suction surface. The shrouds are located at a radial location between the blade dovetail and the blade tip. In some other designs, the shrouds may be located at the tip of the blade airfoil. During normal operation of the compression system, the blades twist and the shrouds on adjacent blades contact with each other, forming a shroud ring that provides support to the blades. During engine operation, the shroud ring resists vibration and twisting of the blades. The term “midspan shroud” is used herein to refer to all supports on fan and compression system blades that contact with each other during operation, and includes all supports located anywhere on the span of the blade, including supports at the tip of the blade. The “midspan shrouds”, as used herein, may be located anywhere along the blade span, not just at the midpoint of the span.
During certain abnormal events, such as a bird impact, other foreign object impact, or stalls during engine operation, the normal contact between the shrouds of adjacent blades is disturbed. The contact forces become high and misaligned due to the impacts and the shrouds become disengaged fully or partially. This is called “shingling” of the blades. Shingling causes significant wear and tear damage on the midspan shrouds. When the speed of the compressor rotor drops, the shingled blades may rebound, causing further wear and tear on the shrouds.
Conventional blades sometimes have wear pads brazed on the contact faces on the midspan shrouds. Wear pads 51, 52, such as shown in FIG. 4 have been used in conventional blades 24 to address the wear problem. For example, some compressor blades made from Titanium contain a brazed on WC—Co wear pad to prevent adhesive wear between two rubbing Ti-6Al-4V midspan shrouds. The wear pads are conventionally brazed to the titanium blade using a titanium-copper-nickel (TiCuNi) alloy braze foils. Diffusion occurs between TiCuNi braze foil and WC—Co wear pad during high temperature braze. Titanium forms brittle compounds with the alloying elements of the wear pad in the braze joint. As a result, the braze joint achieves high hardness, loses ductility and may not be able to withstand impact and tear type failures. TiCuNi braze foil has been found to lack ductility for impact and tear type failures during a shingling event. Conventional pad/braze systems such as shown in FIGS. 4, 5 and 6 (items 51, 52, 53, 54) are unable to sustain pulling loads 105, 106 and rubbing loads 103, 104 on the exposed pad edges that occur when the blades rebound from a shingling event. Both this type of loading and impact damage result in significant loss of wear pad 51, 52 material from the midspan shroud surface and require servicing to restore the wear resistance to the blades.
It is desirable to have a durable blade with wear pads that resist impact and tear type of damage, and further resist such damage when the compression system blades rebound from shingling. It is desirable to have manufacturing methods to apply the wear pads and brazing in such a way that the blades are durable. It is desirable to have repair methods to apply the durable wear pads to blades that have been used in service.